1. Field of the Invention
The present invention relates to a method for improving the filterability and injectivity of aqueous solutions of biopolymers used as thickening agents in enhanced oil recovery processes.
2. Description of the Prior Art
The use of water injection or waterflood operations to recover oil from subterranean reservoirs is a well known and commonly employed practice in the petroleum industry. A typical waterflood comprises injecting an aqueous flooding medium, e.g. water, into the reservoir to drive oil through the reservoir toward one or more production wells from which it can be collected at the surface of the earth. Unfortunately, the injected water tends to channel through certain portions of the reservoir. This inability of water to sweep a substantial percentage of the volume of the reservoir within the pattern of the wells employed in the waterflood operation seriously affects the ultimate recovery of oil, and detracts from the ecnomoic attractiveness of the operation. Accordingly, there is a need to improve the sweep efficiency of waterfloods and this need has long been recognized by persons working within the oil industry.
It is taught in the art that poor sweep efficiency is a result of several factors. One is the natural tendency of the liquid to flow in the path of least resistance; hence, water flows through the highly permeable portions of the heterogeneous reservoir more readily than through the less permeable portions. Another factor involves the difference between the mobilities of the injected water and the oil present in the reservoir. In both instances, sweep efficiency of a reservoir can be improved by increasing the viscosity of the injected water.
The use of hydrophilic, viscosity-increasing additives for flood water is known and practiced in the art, and commonly employed additives for this purpose include partially hydrolyzed polyacrylamides, copolymers of acrylamide and acrylates, and one of the very promising groups of thickeners, ionic polysaccharides, particularly the polysaccharide prepared by employing bacteria of the genus Xanthomonas, the most common of which is referred to as polysaccharide B-1459.
While polysaccharides exhibit many advantageous performance characteristics, and are preferred over other hydrophilic polymers in many applications, certain problems have been identified which limit their effectiveness, at least in certain reservoirs. The most effective and desirable polysaccharides from the standpoint of developing high viscosity at relatively low concentration levels, are somewhat difficult to disperse completely in relatively saline environments, e.g. in field brines containing more than about 50,000 parts per million total dissolved solids.
A major problem in using heteropolysaccharides as thickening agents, even after being dispersed into solution, is that the solutions sill exhibit poor injectivity into the reservoir formation. Injectivity impairment is known to be caused by poor dispersion which leads to plugging of the sandface. For this reason, the major manufacturers of heteropolysaccharides advise passing the biopolymer solutions through a series of shear plates to disperse the biopolymer better. For example, in Technical Bulletin XF#5, published by Kelco, a Division of Merck & Co., Inc., a precise specification for shearing a Xanthamonas-type heteropolysaccharide is disclosed.
One approach to solving the problem of poor injectivity in an especially hostile reservoir environment containing water soluble borates is disclosed in U.S. Pat. No. 4,232,739 to Franklin on Nov. 11, 1980. As part of a process for dispersing polysaccharides in brine, a double shearing procedure is advocated. The particular shear conditions in each of the shear steps are those advanced by Kelco, although Kelco does not specify a double shearing procedure. The patent notes that the shear differential pressure across the shear plates should be maintained below the point which causes degradation of the polymer. The resulting polysaccharide solutions are evaluated for injectivity by passing them through a 3 micron filter, although the data are incomplete because the rates at which the solutions pass through the filter are not given.
Another approach is simply to filter or clarify the solution following conventional shearing. While this may give solutions having excellent filterability, the resulting filtrate may not have the requisite viscosity simply because much of the polymer is filtered out. Moreover, such a process is usually time-consuming and wasteful.
A serious problem still exists, therefore, with preparing biopolymer solutions having good reservoir injectivities. In many cases, better injectivities are required than those possible with a solution which passes through a 3 micron filter. Yet, even repeated passes through shear plates do not yield a solution which has the required injectivity and suitable viscosity.